Direct Observation of Enhanced Raman Scattering on Nano-Sized ZrO₂ Substrate: Charge-Transfer Contribution

Direct observation of the surface-enhanced Raman scattering (SERS) of molecules adsorbed on nano-sized zirconia (ZrO₂) substrates was first reported without the need for the addition of metal particles. It was found that ZrO₂ nanoparticles can exhibit unprecedented Raman signal enhancements on the order of 10³ for the probe molecule 4-mercaptobenzoic acid (4-MBA). The dramatic effect of the calcination temperature on the ZrO₂ nanoparticles was also investigated. The ZrO₂ nanoparticles with the particle diameter of 10.5 nm, which were prepared by calcination at a temperature of 500°C, have the highest SERS activity. A comparison between the experimental and calculation results indicates that charge transfer (CT) effects dominate the surface enhancement. The plentiful surface state of ZrO₂ active substrate that is beneficial to CT resonance occurs between molecules and ZrO₂ to produce a SERS effect. The CT process depends, to a large extent, on the intrinsic properties of the modifying molecules and the surface properties of the ZrO₂. This is a new SERS phenomenon for ZrO₂ that will expand the application of ZrO₂ to microanalysis and is beneficial for studying the basic properties of both ZrO₂ and SERS.